Ugh. Ugly great crash of concentration, memory and motivation. I know it won't last so I'm bumbling along. Went to my piano teacher the other day and could not play a simple short piece that I've played in front of him (and many more people) several times.
I put it down to 'christmas excess'... but I know it's the cognitive effects of brain rebalancing... banks of brain fog.
Still, I know it will disappear eventually.